Fractional HARQ re-transmission

ABSTRACT

Methods and apparatus are described for determining, via a Hybrid Automatic Repeat Request (HARQ) module, that a maximum number of retransmissions has been reached for a HARQ packet. The HARQ module may communicate an internal NACK to a message retransmission module indicating a transmission failure. The message retransmission module may retransmit at least a part of the message. The retransmission may be performed prior to the expiration of a timer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/497,458, filed Jul. 2, 2009, which claims the benefit of U.S.Provisional Application No. 61/078,273, filed Jul. 3, 2008. All of theabove-referenced applications are incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to the field of wireless communications. Moreparticularly, the invention relates to error control in a wirelesscommunication system.

BACKGROUND OF THE INVENTION

In a wireless communication system, the most precious resource, in termsof both capital cost and system capacity, is often the wireless linkitself. Thus, it is important to use the wireless link resourcesefficiently.

BRIEF SUMMARY OF THE INVENTION

In general HARQ operates with an all or nothing signaling mechanism. Inother words, upon receipt of a packet, the receiver indicates to thetransmitter whether the packet was successfully received or whether anerror occurred, i.e. the packet was not successfully received. When anerror occurs, a retransmission is executed whereby the entire originalmessage is resent in its entirety. If the sum of the energy received inthe first transmission and the energy received in the secondtransmission is much larger than is needed to successfully decode thedata, precious wireless link resources have been used with no systemperformance gain. In fact, the additional bandwidth consumed andinterference caused by retransmission may actually degrade theperformance of other links in the system.

With fractional HARQ signaling, a mechanism is provided to allow theretransmission to more closely match the receiver's need for energy withthe additional energy sent over the wireless link. In one aspect, thereceiver sends the transmitter qualitative feedback which indicates tothe transmitter an approximate amount of additional energy that thereceiver needs to successfully decode the transmission. Rather thanretransmit the original message at the same signal level, thetransmitter may send a lesser amount of energy or a smaller message orboth, thereby enhancing the overall system performance and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional block diagram of an embodiment of awireless communication system.

FIG. 2 is a block diagram illustrating a downlink service flowconfiguration that can be implemented in the system of FIG. 1 whereinone embodiment of the fractional HARQ re-transmission is implemented.

FIG. 3 is a block diagram illustrating an uplink service flowconfiguration that can be implemented in the system of FIG. 1 whereinone embodiment of the fractional HARQ re-transmission is implemented.

FIG. 4 is a simplified flowchart showing physical layer operation withinthe base station for the downlink (DL) transmission wherein oneembodiment of the fractional HARQ re-transmission is implemented.

FIG. 5 is an exemplary flow chart showing MAC layer operation at thebase station in the DL transmission wherein one embodiment of thefractional HARQ re-transmission is implemented.

FIG. 6 is an exemplary flow chart showing physical layer operation onthe client station or transmitting station during the uplink (UL)operation wherein one embodiment of the fractional HARQ re-transmissionis implemented.

FIG. 7 is an exemplary flow chart showing physical layer operation onthe base station (receiving station) during the UL operation wherein oneembodiment of the fractional HARQ re-transmission is implemented.

FIG. 8 is an exemplary flow chart showing MAC layer operation at theclient station (transmitting station) during the UL operation whereinone embodiment of the fractional HARQ re-transmission is implemented.

FIG. 9 is a signal flow diagram showing one embodiment of fractionalHARQ processing within the receiving device.

FIG. 10 provides an exemplary block diagram illustrating one embodimentof an apparatus implementing fractional or adaptive HARQ operation.

DETAILED DESCRIPTION OF THE INVENTION

The physical (PHY) layer is the lowest layer protocol layer in acommunication system. It provides the means of transmitting raw bitsover the communication link. In a wireless system, the PHY layerprovides an interface between the medium access layer (MAC) and thewireless link. It performs such functions as electromagnetic spectrumfrequency allocation, specification of signal strength and the like. Italso provides the modulation and coding scheme, forward error correctionand the like.

At the transmitting station, the PHY layer receives MAC packet dataunits (PDUs) from the MAC layer. Typically the MAC PDUs are smaller thanthe largest available PHY layer packet. As such, the PHY layer at thetransmitting station may combine multiple MAC PDUs into one PHY packetbefore transmitting the PHY packet over the wireless link. The PHY layerat the receiving station extracts the corresponding MAC PDUs and passesthem to the MAC layer in the receiving station.

The MAC layer provides addressing and channel access control mechanismsthat make it possible for several client stations to communicate with abase station. The MAC layer is typically largely unaware of the PHYlayer operation and, as such, a common MAC layer may be used in systemsusing disparate PHY layer techniques. For example, the MAC layer isunaware of the packing of multiple MAC PDUs into one PHY packet at thePHY layer.

Automatic Repeat-Request (ARQ) is an error control method for datatransmission. ARQ can be applied to either the MAC layer or the PHYlayer. It uses acknowledgments (ACKs), negative acknowledgments (NACKs)and timeouts to achieve reliable data transmission. An acknowledgmentindicates that a receiving station has correctly received a MAC PDU orPHY packet. A negative acknowledgment indicates that the receivingstation was unable to properly receive a MAC PDU or PHY packet. Atimeout is a time counter that is activated when the transmittingstation sends the PDU or packet and expires at the latest point in timeat which the transmitting station reasonably expects to receive an ACKfrom the receiving station. If the transmitting station does not receivean acknowledgment before the timeout expires, it usually re-transmitsthe PDU or packet until it receives an acknowledgment or a predefinednumber of re-transmissions have occurred.

Hybrid ARQ (HARQ) is a variation of ARQ which has better performance,particularly over wireless channels, at the cost of increasedimplementation complexity. HARQ can be applied to the PHY layer only.HARQ also uses ACKs or NACKs as receiving or non-receiving indicators.Some HARQ protocols also use timeouts. However, according to HARQoperation, when a receiving station fails to properly receive a packet,it saves the energy associated with the failed transmission and combinesit with the energy received in subsequent transmissions of the samepacket.

Forward Error Correction (FEC) is a system of error control for datatransmission, whereby the transmitter adds redundant data to thewireless link message according to a predetermined algorithm. Theredundant data allows the receiver to detect and correct errors, withinsome bound. Each redundant bit is a complex function of many originalinformation bits. The original information may or may not appear in theencoded output. Codes that include the unmodified input in the outputare called systematic, while those that do not are called nonsystematic.Using FEC, a receiver can accurately reproduce the transmittedinformation, even if some PHY layer errors occur. In fact, the receivercan accurately reproduce the transmitted information even if only aportion of the PHY layer bits are transmitted.

Convolution Turbo Code (CTC) is a type of error-correcting encoding usedin FEC in which (a) each m-bit information symbol to be encoded istransformed into an n-bit symbol, where m/n is the code rate (n≧m) and(b) the transformation is a function of the last k information symbols,where k is the constraint length of the code.

Signal-to-noise ratio (often abbreviated SNR or S/N) is defined as theratio of a signal power to the noise power corrupting the signal.Usually SNR represents the power ratio between a signal (meaningfulinformation) and the background noise. The higher the ratio, the lessobtrusive the background noise is.

The carrier-to-interference-and-noise ratio (CINR) is the quotient(C/(N+I)) between the average received modulated carrier power (C) andthe average received co-channel interference power (I) received fromother transmitters summed with the environmental noise (N), such asthermal noise or noise from outside the system. The CINR ratio is animportant measure in an interference limited systems, such as cellularradio systems where frequency channels are reused to achieve high systemcapacity.

A cyclic redundancy check (CRC) is a type of function that takes asinput a data stream of any length, and produces as output a value of acertain space. The term CRC is often used to denote either the functionor the function's output. A CRC can be used as a checksum to detecterrors introduced into data during transmission. CRCs are popularbecause they are simple to implement in binary hardware, are easy toanalyze mathematically, and are particularly good at detecting commonerrors caused by noise in transmission channels.

A low-density parity-check code (LDPC code) is a forward errorcorrecting code for transmitting a message over a noisy transmissionchannel. While LDPC and other error correcting codes cannot guaranteeperfect transmission, the probability of lost information can be made assmall as desired. LDPC was the first code to allow data transmissionrates close to the theoretical maximum, the Shannon Limit.

FIG. 1 is a simplified functional block diagram of an embodiment of awireless communication system 100. The wireless communication system 100includes a plurality of base stations 110 a and 110 b, each supporting acorresponding service or coverage area 112 a and 112 b. The basestations are capable of communicating with wireless devices within theircoverage areas. For example, the first base station 110 a is capable ofwirelessly communicating with a first client station 114 a and a secondclient station 114 b within the coverage area 112 a. The first clientstation 114 a is also within the coverage area 112 b and is capable ofcommunicating with the second base station 110 b. In this description,the communication path from the base station to the client station isreferred to as a downlink (DL) 116 a and the communication path from theclient station to the base station is referred to as an uplink (UL) 116b.

Although for simplicity only two base stations are shown in FIG. 1, atypical wireless communication system 100 includes a much larger numberof base stations. The base stations 110 a and 110 b can be configured ascellular base station transceiver subsystems, gateways, access points,radio frequency (RF) repeaters, frame repeaters, nodes or any wirelessnetwork entry point.

The base stations 110 a and 110 b can be configured to support anomni-directional coverage area or a sectored coverage area. For example,the second base station 110 b is depicted as supporting the sectoredcoverage area 112 b. The coverage area 112 b is depicted as having threesectors, 118 a, 118 b, and 118 c. In typical embodiments, the secondbase station 110 b treats each sector 118 as effectively a distinctcoverage area.

Although only two client stations 114 a and 114 b are shown in thewireless communication system 100, typical systems are configured tosupport a large number of client stations. The client stations 114 a and114 b can be mobile, nomadic or stationary units. The client stations114 a and 114 b are often referred to as, for example, mobile stations,mobile units, subscriber stations, wireless terminals or the like. Aclient station can be, for example, a wireless handheld device, avehicle mounted device, a portable device, client premise equipment, afixed location device, a wireless plug-in accessory or the like. In somecases, a client station can take the form of a handheld computer,notebook computer, wireless telephone, personal digital assistant,wireless email device, personal media player, meter reading equipment orthe like in may include a display mechanism, microphone, speaker andmemory.

In a typical system, the base stations 110 a and 110 b also communicatewith each other and a network control module 124 over backhaul links 122a and 122 b. The backhaul links 122 a and 122 b may include wired andwireless communication links. The network control module 124 providesnetwork administration and coordination as well as other overhead,coupling and supervisory functions for the wireless communication system100.

In some embodiments, the wireless communication system 100 can beconfigured to support both bidirectional communication andunidirectional communication. In a bidirectional network, the clientstation is capable of both receiving information from and providinginformation to the wireless communications network. Applicationsoperating over the bidirectional communications channel includetraditional voice and data applications. In a unidirectional network,the client station is capable of receiving information from the wirelesscommunications network but may have limited or no ability to provideinformation to the network. Applications operating over theunidirectional communications channel include broadcast and multicastapplications. In one embodiment, the wireless system 100 supports bothbidirectional and unidirectional communications. In such an embodiment,the network control module 124 is also coupled to external entities via,for example, content link 126 and two-way traffic link 128.

In one example, the wireless communication system 100 is configured touse Orthogonal Frequency Division Multiple Access (OFDMA) communicationtechniques. For example, the wireless communication system 100 can beconfigured to substantially comply with a standard system specification,such as IEEE 802.16 and its progeny or some other wireless standard suchas, for example, WiBro, WiFi, Long Term Evolution (LTE) or it may be aproprietary system. The ideas described herein are not limited toapplication to OFDMA systems. The description in the context of an OFDMAsystem is offered for the purposes of providing a particular exampleonly.

Each base station, for example 110 a, can supervise and control thecommunications within its respective coverage area 112 a. Each activeclient station (e.g., 130 a) registers with a base station (e.g., 110 a)upon entry into its coverage area (e.g., 112 a). Typically, the clientstation 130 a can notify the base station 110 a of its presence uponentry into the coverage area 112 a, and the base station 110 a caninterrogate the client station 130 a to determine the capabilities ofthe client station 130 a.

When, for example, the client station 130 a establishes a service flow,such as an Internet connection or a voice connection, with the basestation 110 a, a HARQ MAC layer service flow state machine as well as aHARQ PHY layer service flow state machine are established in both theclient station 130 a and the base station 110 a, as illustrated in FIGS.2-3. The HARQ state machines allow error correction to be applied to thewireless link.

In general HARQ operates with an all or nothing signaling mechanism fromthe receiver indicating that the full packet was successfully receivedor that a full retransmission is required. With a fractional ACK/NACKsignal, a mechanism can be provided where a receiver can describe thelevel of retransmission that should be required to successfully completea HARQ transmission.

The fractional return signal can be generated locally by a receiverbased on metrics generated by the decoders that were performing ForwardError Correction (FEC) on the receiver information. In particular whiledecoding Convolution Turbo Coded (CTC) or LDPC FEC blocks, qualitymetrics from the internal operation of the decoder can be extracted.Another metric might be the instantaneous SNR measured while receivingthe HARQ packet and the accumulated CINR so far for the HARQ packet. TheUL control channel for HARQ can be designed to support multiple levelsignaling, for example, a 3 or 4 level signaling.

The benefits of the fractional feedback signaling come from the reducedretransmission rate and come at the cost of increased signalingoverhead. However, even with the increased complexity in management andsignaling associated with fractional HARQ, there can be an overall netthroughput gain of about 10%-20%.

A full HARQ channel of information contains a user data portion and aCRC checksum. The CRC checksum values for a HARQ channel must pass forthe receiver to signal an “ACK” back to the transmitter. An ACK'dchannel can be considered to be closed and the channel can now be usedby the transmitter for new information.

If the Checksums do not pass, a “NACK” is signaled back to thetransmitter. A NACK may be signaled expressly or implicitly. The NACKindication can be interpreted by the transmitter as requesting aretransmission of the information for that HARQ channel. In thereceiver, each subsequent retransmission is summed with the storedresult for that HARQ channel at the receiver and the same checksumpass/fail condition is applied resulting in another ACK/NACK signal backto the transmitter.

With reference to FIGS. 2, 4 and 5, an error control method in the DLoperation according is described. FIG. 2 is a block diagram illustratinga downlink service flow configuration 200 for error control from boththe base station and client station perspectives. FIG. 4 is a simplifiedflow chart showing DL PHY layer operation 400 in the base station. FIG.5 is a flow chart showing DL MAC layer operation 500 at the basestation.

As seen in FIG. 2, for DL operation, the MAC layer of the base stationcomprises an application layer 210 configured to send one or moreservice data units (SDU) to a fragmentation machine 212 where a seriesof MAC PDUs are created based on the received SDUs. The base stationalso comprises a MAC layer re-transmission state machine 214 thatreceives the MAC PDUs from the fragmentation machine and sends to a HARQstate machine 216 in the PHY layer 218 from which the MAC PDUs are sentto the client station. On the other hand, in the client station side,there is also a PHY layer 220 and HARQ state machine 222 from which theMAC PDUs are sent to a reassembly machine 224. The reassembly machine224 then processes and reassembles the PDUs into SDUs to pass onto theapplication layer 226. During the above-mentioned transmission of datapackets and interaction of each component, certain errors may occur.Below is described an exemplary DL service flow including error controlat the base station.

The service flow starts in block 510 of FIG. 5. In block 512, thefragmentation machine 212 receives a service data unit (SDU) from theapplication layer 210. The fragmentation machine 212 creates a series ofMAC PDUs in block 514 which are received by the MAC layer retransmissionmachine 214. Each PDU is assigned a unique service flow identifier(SFID) depending on its associated service or data connection. Forexample, if the client station is participating in a voice call whilesurfing the Internet, each of the voice and data connections is assigneda unique service flow identifier (SFID). In practice, separate MAC layerretransmission state machines (e.g., #a . . . , #x) can be used tofurther transmit the MAC PDUs to the HARQ state machine 216. In doingso, each MAC layer retransmission state machine 214 (e.g., #a, . . . ,#x) will send one or more MAC PDUs of the same SFID to the HARQ statemachine 216 through a different connection or service flow.

In block 518, the MAC layer retransmission state machine 214 saves acopy of the MAC PDUs for possible full or fractional retransmission incase of transmission error or failure. In block 520, the MAC layerretransmission state machine 214 determines whether an internal NACK230, which may be a multi-level NACK, was received from the HARQ statemachine 216. If not, after a predetermined time has expired, the serviceflow for this particular MAC PDU ends assuming it has been successfullyreceived by the client station, although the general process maycontinue for other MAC PDUs associated with the SDU. When the timerexpires, the MAC layer retransmission state machine 214 no longer needsto save this particular PDU for possible retransmission.

In block 520, if an internal NACK is received from the HARQ statemachine before the timer expires, the service flow with regard to theparticular MAC PDU continues to block 522 in which the PDU-associatedMAC layer retransmission state machine 214 determines whether a maximumnumber of retransmissions has occurred with respect to the current PDU.If not, the flow continues to block 528 in which the retransmissioncounter is incremented, and may be incremented by a fractional amount.In block 530, the MAC layer retransmission state machine 214 resends theMAC PDU or portion specified by the fractional HARQ. If in block 522 themaximum number of retransmissions has been reached, the MAC layerretransmission state machine 214 discards this particular MAC PDU inblock 524 and flow ends in block 526 for the particular MAC PDU,although the general flow may continue for other MAC PDUs associatedwith the SDU. In such a case, the application 210 will also experienceerror.

Referring now to FIG. 4, the DL PHY layer flow at the base stationstarts in block 410. In block 412, the HARQ state machine 216 receivesthe MAC PDU, such may have been sent by the MAC layer retransmissionstate machine 214 in either block 516 or 530 of FIG. 5. In oneembodiment, the HARQ state machine 216 is capable of combining multipleMAC PDUs into one HARQ packet, as shown in block 414. In block 416, thecore physical layer 218 transmits the HARQ packet over the wireless linkto the client station. The HARQ state machine 216 awaits a response fromthe HARQ state machine 222 within the client station. If the HARQ statemachine 216 receives any indication of NACK in block 418, the flowcontinues to block 420, otherwise the HARQ packet is ACKed and the flowproceeds to block 428.

In block 420, the HARQ state machine 216 determines whether a maximumnumber of retransmissions has been exceeded. If not, the flow continuesto block 422 where the retransmission error count is incremented, andmay be incremented by a fractional value. Subsequently, the HARQ statemachine 216 retransmits the HARQ packet or fractional portion indicatedby a multi-level acknowledgement to the client station, as the flow goesback to block 416.

If in block 420 the maximum number of retransmissions has been reached,the flow continues to block 424 in which the HARQ state machine 216sends an internal NACK 230 to the MAC layer retransmission state machine214 to indicate transmission error of the MAC PDUs and the flow ends inblock 426. The creation of the internal NACK by the HARQ state machine216 based on the physical layer error correction mechanisms obviates theneed for the transmission of a MAC NACK over the wireless link, thuspreserving the precious wireless link resources.

If in block 418, no HARQ NACK is received before a timer within the HARQstate machine 216 expires, the flow continues to block 428. In block428, the HARQ state machine 216 sends an internal ACK 230 to MAC layerretransmission state machine 214. The creation of the internal ACK bythe HARQ state machine 216 based on the physical layer error correctionmechanisms obviates the need for the transmission of a MAC ACK over thewireless link, thus preserving the precious wireless link resources.

In FIG. 2, on the client station side, the core physical layer 220responds in a HARQ manner sending error indications and good HARQpackets to the HARQ state machine 222. The error indications may requestretransmission of an entire HARQ packet or only a fraction of the HARQpacket. In the operation as described in FIGS. 4 and 5, onlysuccessfully received MAC PDUs are passed from the HARQ state machine tothe upper layers. Thus, the client station need not include a MAC layerretransmission state machine and the MAC PDUs from the HARQ statemachine 222 can be passed directly to the reassembly state machine 224.The reassembly machine 224 reassembles the SDU and passes it to acorresponding application layer 226.

Although the HARQ ACK/NACK external signaling 232 is shown as flowingfrom the HARQ state machine 222 in the client station to the HARQ statemachine 216 in the base station directly, according to industry standardpractice in such a representation, the HARQ ACK/NACK message istypically transmitted via the core physical layers 218, 220.

As generally illustrated in FIGS. 2, 4, and 5, in the DL, the basestation is the transmitter and the client station is the receiver. TheBS relies on HARQ ACK/NACKs to drive MAC level retransmissions (ARQ).The BS keeps track of MAC PDUs mapping to HARQ packets and whetherseveral MAC flows are multiplexed onto the same HARQ packet. The ARQstate machine can be advanced based on HARQ ACK/NACK. A window size ofthe NACK based ARQ can be managed using the Fragmentation SequenceNumber (FSN).

If the HARQ packet is ACK'ed by the client station, the BS sends aninternal ACK to the ARQ state machine for the associated MAC PDU. If theHARQ process is terminated with an unsuccessful outcome (independent ofthe maximum HARQ retransmission parameter), the BS sends an internalNACK indication for the associated PDUs. HARQ packets may carry multipleMAC flows. In this case, the base station may send internal ACK/NACKindications to multiple MAC flows state machines.

Referring now to FIGS. 3, 6-8, an exemplary UL flow between the clientstation and base station with improved error control mechanisms isherein described in detail. FIG. 3 is a block diagram illustrating an ULservice flow configuration 300 from both base station and client stationperspectives. FIG. 6 is an exemplary flow chart showing UL PHY layeroperation 600 in the client station (transmitting station). FIG. 7 is anexemplary flow chart showing UL PHY layer operation 700 on the basestation (receiving station). FIG. 8 is an exemplary flow chart showingUL MAC layer operation 800 at the client station (transmitting station).

As shown in FIG. 3, for UL operation, the client station comprises anapplication layer 330 that sends SDUs to a fragmentation state machine328 where a series of MAC PDUs are created. The client station alsocomprises a MAC retransmission state machine 324 that receives MAC PDUsfrom the fragmentation state machine 328 and then sends them to a HARQstate machine 322 and a core PHY layer 320. On the base station side, inthe PHY level there are a core PHY layer 316 and HARQ state machine 314in communication with the client station. The base station alsocomprises a MAC layer retransmission state machine 312, a reassemblystate machine 310 receiving MAC PDUs from the HARQ state machine 314 andreassembling them into SDUs, and an application layer 308 receiving SDUsfrom the reassembly state machine 310. Detailed transmission andinteraction between the aforementioned components are described below,referring to the UL service flows in FIGS. 6-8.

The UL service flow starts at the MAC layer of the client station inblock 810 of FIG. 8. In block 812, in the client station thefragmentation state machine 328 receives a service data unit (SDU) fromthe application layer 330. In block 814, the fragmentation state machine328 creates a series of MAC PDUs based on the received SDU and providesthem to the MAC retransmission state machine 324. In block 816, the MACretransmission state machine 324 sends a MAC PDU to the HARQ statemachine 322. In block 818, the MAC retransmission state machine 324saves the MAC PDU value for possible retransmission, either as a fullHARQ packet or a fractional HARQ packet. In block 820, the MACretransmission state machine 324 sets a timer and monitors whether aNACK message is received before the timer expires. If a NACK is notreceived before the timer expires, the MAC retransmission state machine324 assumes that this particular MAC PDU was successfully deliveredacross the wireless link to the base station and the flow ends for theMAC PDU of interest in block 822, although the general process maycontinue with respect to other MAC PDUs corresponding to the SDU.

If in block 820 a NACK is received before the timer expires, the flowcontinues to block 824 where the MAC retransmission state machine 324determines whether a retransmission count has reached a maximum value.If not, the flow continues to block 826 in which the retransmissioncounter is incremented by an amount corresponding to the fractionalportion, up to one, of the HARQ packet that is to be retransmitted. Inblock 828, the MAC retransmission state machine 324 resends the MAC PDUor fractional part of it to the HARQ state machine 322 and PHY layer320.

Then the UL service flow continues to the PHY layer of the clientstation in start block 610 of FIG. 6. In block 612, the HARQ statemachine 322 receives one or more MAC PDUs from the MAC retransmissionstate machine 324. The HARQ state machine 322 puts one or more MAC PDUsinto an HARQ packet in block 614. In block 616, the HARQ state machine322 awaits a grant of an uplink allocation from the base station. Whenthe allocation is granted, the HARQ start machine 322 re-sets aretransmission counter in block 618. In block 620, the HARQ statemachine 322 passes the HARQ packet to the core physical layer 320 whichtransmits it over the wireless link to the core physical layer 316 ofthe base station. The retransmission counter value is incremented by theHARQ state machine 322 in block 622 by the fractional amount, up to one.

In block 624, the HARQ state machine 322 determines whether a maximumretransmission counter value has been reached. If not, the flowcontinues to block 626 in which the HARQ state machine 322 determineswhether a subsequent allocation grant is received for retransmission ofthe HARQ packet, which provides an implicit message whether the HARQpacket has been delivered successfully. This is because when the maximumnumber of retransmission has not been reached in this case, the receiptof a grant specifying the transmission of new information is aconfirmation that the previous HARQ packet was properly received and,thus, is an implicit ACK. The receipt of a grant specifying a request torepeat previously sent information is an indication that the previousHARQ packet was not properly received and, thus, is an implicit NACK.Alternatively, the HARQ state machine 322 receives an explicit ACK orNACK message from the base station (not shown in FIG. 6) so as to decidewhether retransmission is necessary.

Referring back to block 626, if an allocation for retransmission isreceived, the flow proceeds to generate a bandwidth optimized HARQretransmission, as specified by a multi-level NACK. Flow then goes backto block 620 where the HARQ state machine 322, again, passes the HARQpacket to the core physical layer 320 which transmits it over thewireless link to the core physical layer 316 of the base station.Conversely, if the allocation grant specifies the transmission of newinformation, the flow continues to block 628 in which the HARQ statemachine 322 creates an internal pending ACK 336. In block 629, the HARQstate machine 322 sets a timer. If the timer expires before a MAC NACK342 is received from the MAC layer retransmission state machine 312, theHARQ state machine 322 sends the internal ACK 336 to the MACretransmission state machine 324 and the flow ends in block 630 for theHARQ packet of interest. On the other hand, if the MAC NACK 342 isreceived before the timer expires, the HARQ state machine 322 eithersends an internal NACK 336 to the MAC retransmission state machine 324or simply discards the internal pending ACK. Either way, the flow endsin block 630.

If the allocation specifies the retransmission of previously sentinformation, flow continues to block 620 and the processes of blocks620, 622, 624 are repeated until the retransmission counter value (E)exceeds a predetermined value. For example, in one embodiment, the HARQpacket is sent to the base station up to four times. Once theretransmission counter value has reached its maximum value, the HARQstate machine 322 creates an internal pending ACK 336 in block 631. TheHARQ state machine starts a timer in block 632. If the timer expiresbefore receipt of a MAC NACK 342, the HARQ state machine 322 sends theinternal ACK 336 to the MAC retransmission state machine 324. On theother hand, if the MAC NACK 342 is received before the timer expires,the HARQ state machine 322 either sends an internal NACK 336 to the MACretransmission state machine 324 or simply discards the internal pendingACK. Either way, the flow ends in block 634.

On the base station side, the uplink flow in the physical layer beginsin block 710 of FIG. 7. In block 712, the HARQ state machine 314 re-setsa retransmission counter value (E.) In block 714, the HARQ state machine314 receives a good HARQ packet or a failure indication 332 from thecore physical layer 316. If in block 716 a good HARQ packet wasreceived, flow continues to block 718 where an internal ACK message 330is sent to the MAC layer retransmission state machine 312, after whichthe flow ends in block 720 for the MAC PDU of interest. The transmissionof the express internal ACK in block 718 obviates the need for theclient station to send an explicit ACK message over the wireless link,thus preserving the precious wireless link resources.

If in block 716 the HARQ state machine 314 failed to properly receivethe HARQ packet, the flow continues to block 722 in which theretransmission counter value is incremented by a fractional amountrepresenting a fractional retransmission, up to one for a wholeretransmission. In block 724, the HARQ state machine 314 determineswhether a maximum retransmission counter value has been reached. If themaximum retransmission counter value has not been reached, the HARQstate machine 314 requests an uplink allocation over which the clientstation can retransmit the HARQ packet or fractional HARQ packet. Thescheduler (not shown) by way of this request is made aware of the needfor retransmission and, in one aspect, can indicate such to the clientstation by using, for example, the AI_SN toggle bit 340 specified inWiMAX. On the other hand, if in block 724 the maximum retransmissioncounter value has been reached, the state machine 314 sends an internalNACK 330 to the MAC layer retransmission state machine 312, once againobviating the need for such a message to be sent over the wireless link.

The successfully received MAC PDU is passed from the HARQ state machine314 to the reassembly state machine 310. The reassembly state machine310 recreates the SDU based on the MAC PDU and passes it to anapplication layer 308.

FIG. 10 is a block diagram illustrating one embodiment of an apparatusimplementing fractional or adaptive HARQ operation. FIG. 10 illustratesa high-level overview of an operational flow within an apparatus usingone embodiment of a fractional or adaptive HARQ signaling mechanism.

As shown in FIG. 10, a HARQ decision block is introduced to determinethe value of an ACK message based on information such as statisticalinformation from the MAC regarding previous HARQ channel successes andfailures, or the CRC pass/fail metrics for the full HARQ channel andalso partial CRC results that may contain per FEC block CRC values, ordecoder-generated information about the HARQ channel (e.g., per FECblock pass/fail values, per FEC block quality readings computed duringthe decoding process), or accumulated CINR values for the HARQ channel,or a combination of some or all of the above.

In fractional HARQ operation, a receiver can signal a multi-levelACK/NACK signal instead of a single bit. One setting within themulti-level mapping can be used to signal successful reception (an ACK)and another setting can be used to signal that a full retransmission isrequired (a NACK), the other settings within the overall mapping can beused to signal that only a partial retransmission is required tosuccessfully close the currently operating channel.

The receiver can use a variety of different information derived orotherwise obtainable at the receiver to determine the correct adaptiveACK/NACK signal. As discussed above, the information can include, forexample, Accumulated CINR values, Partial CRC completion flags,Qualitative Decoder Feedback, and the like or some combination thereof.

In one embodiment, as each retransmission is received, the receiver candetermine or otherwise compute an accumulated CINR value. In anotherembodiment, where there are several FEC blocks in a HARQ channel, one ormore partial CRC completion flags can be used to indicate that one orseveral FEC blocks in a HARQ channel are successfully received. For manyclasses of decoders such as Turbo codes or LDPC codes a variety ofmetrics can be determined from the internal operation of the decoder.Such feedback can include success/failure determination or qualitativevalues signaling the perceived performance of the decoding.

In addition to the various forms of qualitative feedback describedabove, the following qualitative feedback matrices and features can beused.

1. For accumulated CINR metrics—If the accumulated CINR value indicatesthat the HARQ channel is close to decoding then the next transmissioncan be reduced in size.

2. For updated SNR—In UL HARQ the base station may receive SNR updatesbetween scheduled grants to the UL HARQ transmitter. Based on these SNRupdates, the size of the next re-transmission may be increased ordecreased.

3. For Decoder metrics—This is information generated by FEC decodercircuitry indicating how close to decoding a particular FEC block is.

CRC checks—Main item here is the HARQ channel pass/fail CRC. It is alsopossible that if several FEC blocks are present in the HARQ channel andthat each FEC block may have its own CRC check.

4. Statistics within the MAC over the course of time can indicate thegeneral performance of a HARQ channel. Based on this statisticaldatabase and the current state of the HARQ channel a multi-levelfractional HARQ signal can be generated.

The uplink channel can similarly benefit from the implementation offractional HARQ signaling. For UL reception the situation is verysimilar to the DL case except that the UL receiver, or controller incommunication with the UL receiver, is generally in charge of schedulingresource grants for the UL transmitter. As such, the result of advancedHARQ operation can be included within the control information for thenext scheduling grant for each UL HARQ channel. In addition updated SNRinformation known by the serving base station about the UL transmittercan be used to further improve the fractional HARQ selection for thenext UL transmission.

The HARQ feedback information supports multiple levels of fractionalfeedback. This type of feedback can be provided in both DL and the UL.Table 1 provides examples of several embodiments of multi-levelsignaling for fractional HARQ and the retransmission request associatedwith each signaling level. Although 3-level and 4-level signaling isillustrated in Table 1, a larger number of signaling levels can be usedand may be advantageous with increasing block size.

TABLE 1 Fraction ACK/NACK Signal Formats Number of Levels LevelInterpretations 3 0 NACK, Transmit Full amount of data 1 Half_ACK,Transmit only ½ of the normal amount of data on the next TX 2 Full_ACK,Transmit success 4 0 NACK - Transmit Full amount of data 1Partial_ACK1 - Transmit ½ of normal 2 Partial_ACK2 - Transmit ¼ ofnormal 3 Full_ACK - transmit success 4 0 NACK - Transmit Full amount ofdata 1 Partial_ACK 1 - Transmit ⅔ of normal 2 Partial_ACK 2 - Transmit ⅓of normal 3 FullACK - transmit success

FIG. 9 shows a detailed signal flow diagram using one embodiment offractional or adaptive HARQ signaling mechanism. As seen in FIG. 9,various signal metrics can be used for HARQ signaling, such asaccumulated CINR metrics, updated SNR, certain decoder metrics and soforth. Specifically, For accumulated CINR metrics, if the accumulatedCINR value indicates that the HARQ channel is close to decoding, thenthe next transmission can be reduced in size. For updated SNR, in ULHARQ the base station may receive SNR updates in between schedule grantsto the UL HARQ transmitter, and based on these SNR updates the size ofthe next re-transmission may be increased or decreased. For decodermetrics, i.e., information generated by CTC decoders, indication can bemade, calculated, or otherwise generated to reflecting a measure ofprogress, for example, as to how close to decoding a particular FECblock is.

When multiple HARQ FEC blocks are allowed in one allocation (or calledPHY PDU) in order to reduce allocation signaling (i.e., MAP) overhead,the fractional HARQ ACK/NACK can be implemented as a two-level scheme. Afirst level (level-one) can include a per-FEC block ACK/NACK, whereas asecond level (level-two_can include, inside one FEC block, ACK/NACK moreinformation to enable the transmit-side to make more intelligentdecisions for adaptive retransmission.

In one particular embodiment of the per-FEC block ACK/NACK there may bea maximum of 8 or 12 FEC blocks per PHY PDU (one allocation). A 24-bitCRC may be implemented for each FEC block. A bit-map mechanism can beused to ACK/NACK FEC blocks to enable retransmitting only the corruptedblocks. Level-two ACK/NACK can be considered for those corrupted blocks,e.g., one bit for the good blocks, multiple bits for the bad blocks.

In UL HARQ, the base station can transmit the bit-map ACK/NACK asunicast data to the client station. In the complementary DL HARQ, it canbe more challenging to pack more bits in the HARQ ACK/NACK channel.Embodiments for supporting the multiple level DL HARQ include thefollowing. As the BS is the transmit side of DL HARQ, it can schedule aunicast UL data allocation for HARQ ACK/NACK and other UL data if any.The system may implement a code for the HARQ ACK/NACK channelsubcarriers with maximally orthogonal messages. For example, a WiMax ULhalf-slot HARQ ACK/NACK channel can carry 6-bit data in a really robustway; 8-bit or even 12-bit in a fairly robust way depending on UL channelconditions. The BS should have relatively good knowledge about the ULchannel condition as it needs the information to schedule the clientstation's UL MCS, therefore, there could be some adaptive ways to tiethe number of bits and number of max FEC blocks per PHY PDU.

Embodiments of the actual UL ACK/NACK message include, for a ½ slot ACK24 QPSK subcarriers or 48 bits to convey this information. For full slotfeedback there can be 48 subcarriers or 96 values. These can be used totransmit 6 bits of information or one of 64 different values. For a ½slot then for equivalent performance 32 values could be represented. Ingeneral, the ACK/NACK should be somewhat robust unless the channelconditions are known to be good.

Examples of possible mappings for FEC blocks and the associatedmulti-level fractional HARQ signaling include: For one FEC blockimplement a 3 valued signal (ACK, ½ACK and NACK); For two FEC blocksimplement a 9 valued signal (3 valued signal for each FEC block); Forthree FEC blocks have a 27 valued signal, similar to the 32 values forhalf-slot fast feedback as mentioned above. Reasonable channel qualityexists in this case; For four FEC blocks move to one bit per FEC block(4 bits or 16 valued signal); For more FEC blocks (up to 8 as a max)implement the one bit per FEC block.

In the 4 FEC block or more case, the per FEC block CRC flag may be usedto generate the feedback.

As the number of FEC blocks in the HARQ channel increases, the channelquality is typically also increasing, generally meaning that more UL ACKinformation can be transmitted in the same space. For example, acommunication link between two wireless devices using an 8 FEC blockHARQ channel can typically tolerate an 8 bit feedback using QPSK and a ⅙coding.

In operation, with the fractional HARQ signal mechanism as demonstratedin FIGS. 9-10, the system (from either the base station or clientstation perspective, and in either DL or UL operation) can beimplemented and work as illustrated in FIGS. 2-8 according to variousembodiments.

In one aspect, incremental redundancy (IR) HARQ is used for fractionalHARQ signaling. IR HARQ makes use of the fact that by using FEC, areceiver can accurately reproduce the transmitted information, even ifsome PHY layer errors occur and even if only a portion of the PHY layerbits are transmitted. Instead of sending an entire FEC block, initiallyonly a portion of a FEC block is transmitted, thereby using lesswireless link resources. According to one implementation, theretransmitted portion is configured to be sufficient for decoding theblock at a desired probability level. Alternatively or additionally, ifthe block was not received properly, instead of sending a simple repeatof the same portion, progressive parity packets are sent in eachsubsequent transmission of the packet. The decoder then combines all thetransmissions and decodes the packet at a lower code rate.

The IR schemes can be broadly classified under two categories: a)partial IR where each re-transmission is self decodable (referred to asHARQ-type-III) as described above and b) full IR where each transmissionis not self decodable (referred to as HARQ-type-II) and an additionalretransmission is required. Fractional HARQ signaling can be used witheither HARQ type II or HARQ type III In addition, fractional HARQsignaling can be used with both systematic and non-systematic HARQschemes.

With fractional HARQ signaling, a mechanism is provided to allow theretransmission to more closely match the receiver's need for energy withthe additional energy sent over the wireless link. In one aspect, thereceiver sends the transmitter qualitative feedback which indicates tothe transmitter an approximate amount of additional energy that thereceiver needs to successfully decode the transmission. Rather thanretransmit the original message at the same signal level, thetransmitter may send a lesser amount of energy and/or a smaller messagebased on the qualitative feedback. The reduced transmission conservesthe precious wireless link resources for use by other links.

A mechanism for fractional HARQ signaling and apparatus for fractionalHARQ signaling are described herein.

As used herein, the term coupled or connected is used to mean anindirect coupling as well as a direct coupling or connection. Where twoor more blocks, modules, devices, or apparatus are coupled, there may beone or more intervening blocks between the two coupled blocks.

The steps of a method, process, or algorithm described in connectionwith the embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. The various steps or acts in a method or processmay be performed in the order shown, or may be performed in anotherorder. Additionally, one or more process or method steps may be omittedor one or more process or method steps may be added to the methods andprocesses. An additional step, block, or action may be added in thebeginning, end, or intervening existing elements of the methods andprocesses.

The methodologies described herein may be implemented by various meansdepending upon the application. For example, these methodologies may beimplemented in hardware, firmware, software, or a combination thereof.For a hardware implementation, the processing units may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof.

For a firmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. Any machine readable mediumtangibly embodying or otherwise encoded with instructions may be used inimplementing the methodologies described herein. For example, softwarecodes may be stored or otherwise encoded in a memory and executed by aprocessor. Memory may be implemented within the processor or external tothe processor. As used herein the term “memory” refers to any type oflong term, short term, volatile, nonvolatile, or other memory and is notto be limited to any particular type of memory or number of memories, ortype of media upon which memory is stored.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored as one or moreinstructions or code on a computer-readable medium. Computer-readablemedia includes physical computer storage media. Transmission mediaincludes physical transmission media. A storage media may be anyavailable media that can be accessed by a computer. By way of example,and not limitation, such computer-readable media can comprise RAM, ROM,EEPROM, CD-ROM or other optical disk storage, magnetic disk storage orother magnetic storage devices, or any other medium that can be used tostore desired program code in the form of instructions or datastructures and that can be accessed by a computer disk and disc, as usedherein, includes compact disc (CD), laser disc, optical disc, digitalversatile disc (DVD), floppy disk and blu-ray disc where disks usuallyreproduce data magnetically, while discs reproduce data optically withlasers. Combinations of the above should also be included within thescope of computer-readable media.

The above description of the disclosed embodiments is provided to enableany person of ordinary skill in the art to make or use the disclosedembodiments. Various modifications to these embodiments will be readilyapparent to those of ordinary skill in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the scope of the disclosure.

What is claimed is:
 1. A method for use in a transmitting station forerror control, the method comprising: determining, at a Hybrid AutomaticRepeat Request (HARQ) module, that a maximum number of retransmissionshas been reached for a HARQ packet, the HARQ packet representing atleast a portion of a message; communicating, by the HARQ module to amessage retransmission module, an internal NACK indicating atransmission failure; and retransmitting at least a part of the messageby the message retransmission module.
 2. The method of claim 1, whereinthe retransmitting is performed prior to expiration of a timer.
 3. Themethod of claim 2, wherein the timer is associated with retransmissionof the message.
 4. The method of claim 1, wherein the retransmitting atleast a part of the message comprises retransmitting the message.
 5. Themethod of claim 4, wherein the internal NACK indicates the part of themessage to be retransmitted.
 6. The method of claim 1, wherein theretransmitting by the retransmission module is performed on a conditionthat a retransmission counter at the retransmission module does notexceed a predetermined threshold for a maximum number ofretransmissions.
 7. The method of claim 6, further comprisingincrementing, at the message retransmission module, the retransmissioncounter.
 8. The method of claim 1, wherein the internal NACK is amulti-level NACK.
 9. The method of claim 1, wherein the transmittingstation is a base station.
 10. The method of claim 1, wherein thetransmitting station is a client station.
 11. A transmitting station forerror control, the transmitting station comprising: a Hybrid AutomaticRepeat Request (HARQ) module configured to: determine that a maximumnumber of retransmissions has been reached for a HARQ packet, the HARQpacket representing at least a portion of a message; and communicate, toa message retransmission module, an internal NACK indicating atransmission failure; and the message retransmission module configuredto retransmit at least a part of the message.
 12. The transmittingstation of claim 11, wherein the message retransmission module isfurther configured to retransmit the at least a part of the messageprior to expiration of a timer.
 13. The transmitting station of claim12, wherein the timer is associated with retransmission of the message.14. The transmitting station of claim 11, wherein the messageretransmission module retransmitting at least a part of the messagecomprises retransmitting the message.
 15. The transmitting station ofclaim 14, wherein the internal NACK indicates the part of the message tobe retransmitted.
 16. The transmitting station of claim 11, wherein themessage retransmission module is further configured to retransmit the atleast a part of the message on a condition that a retransmission counterat the retransmission module does not exceed a predetermined thresholdfor a maximum number of retransmissions.
 17. The transmitting station ofclaim 16, wherein the message retransmission module is furtherconfigured to increment the retransmission counter.
 18. The transmittingstation of claim 11, wherein the internal NACK is a multi-level NACK.19. The transmitting station of claim 11, wherein the transmittingstation is a base station.
 20. The transmitting station of claim 11,wherein the transmitting station is a client station.
 21. Anon-transitory computer-readable storage medium containing a set ofinstructions for execution by one or more processors, the set ofinstructions comprising: a first code segment for determining that amaximum number of retransmissions has been reached for a HybridAutomatic Repeat Request (HARQ) packet, the HARQ packet representing atleast a portion of a message; a second code segment for communicating aninternal NACK indicating a transmission failure; and a third codesegment for retransmitting at least a part of the message.
 22. Thenon-transitory computer-readable storage medium of claim 21, wherein thethird code segment is further configured to retransmit the at least apart of the message prior to expiration of a timer.
 23. Thenon-transitory computer-readable storage medium of claim 22, wherein thetimer is associated with retransmission of the message.